1. Field of the Invention
The present invention relates to a surface plasmon sensor which enables quantitative analysis of a specific material in a sample by utilizing generation of a surface plasmon, and in particular, to a surface plasmon sensor whereby a measurement result is not affected by an inclination of the sample.
2. Description of the Related Art
In a metal, free electrons move collectively to produce a compressional wave called a plasma wave. When a plasma wave generated at the surface of the metal is quantized, the plasma wave is regarded as surface plasmons.
The surface plasmons can be produced by exciting a surface of a metal using an optical wave. Conventionally, various surface plasmon sensors are proposed for performing quantitative analysis of a material in a sample by utilizing the excitation by an optical wave. In particular, surface plasmon sensors which use a system called Kretschmann""s arrangement are well known. Refer to Japanese Unexamined Patent Publication No. 6(1994)-167443.
The surface plasmon sensors which use the above system basically contain a prism made of a dielectric material, a metal film formed on a face of the prism and in contact with a sample, a light source producing a light beam, an optical system letting the light through the prism and letting the light beam be incident on a boundary surface between the prism and the metal film at various incident angles, and a light detection unit which can detect the intensity of the light beam reflected by the above boundary surface for each incident angle.
The above various incident angles can be realized by deflecting a relatively thin light beam so that the deflected beam is incident on the boundary surface at a desired incident angle. Otherwise, the various incident angles can be realized by letting a relatively thick light beam be incident on the boundary surface so that the thick light beam converges near or at the boundary surface, and therefore the converging beam contains components incident on the boundary surface at the various incident angles. When deflecting the relatively thin light beam, the light beam reflected at a reflection angle which varies with the deflection of the incident light beam can be detected by a small light detector which moves corresponding to the deflection of the incident light beam, or by an area sensor extending in the direction of the variation of the reflection angle. When letting the relatively thick light beam be incident on the boundary surface as above, the reflected light beam can be detected by an area sensor which extends in the direction of the variation of the reflection angle so that substantially all of the reflected light beam can be detected.
When a light beam is incident on the metal film at a specific incident angle xcex8SP which is greater than a critical angle for total reflection in the surface plasmon sensor having the above construction, an evanescent wave is generated, where an electric field of the evanescent wave spreads out near the metal film in the sample. As a result of the evanescent wave, surface plasmons are generated at the boundary surface between the metal film and the sample. When the wave number of the evanescent wave equals the wave number of the surface plasmons, i.e., these wave numbers match, the evanescent wave is resonant with the surface plasmons, and the energy of the evanescent wave transfers to the surface plasmons. Therefore, the intensity of the light totally reflected by the boundary surface between the prism and the metal film sharply decreases at the above specific incident angle xcex8SP.
When the wave number of the surface plasmon is obtained based on the incident angle xcex8SP (generally called total reflection break angle) at which the above intensity decreases, the permittivity of the sample can be obtained from the wave number of the surface plasmons. That is,
KSP(xcfx89)=(xcfx89/c){square root over (xcex5m+L (xcfx89)xc2x7xcex5s+L /(xcex5m+L (xcfx89)+xcex5s+L ))},
where the wave number of the surface plasmon is denoted by KSP, the angular frequency of the surface plasmon is denoted by xcfx89, the velocity of light in vacuum is denoted by c, and permittivities of the metal and the sample are denoted by xcex5m and xcex5s, respectively.
When the permittivity xcex5s of the sample is obtained, the concentration of the specific material in the sample can be obtained based on a predetermined calibration curve or the like. Therefore, a quantitative analysis of the specific material in the sample can be performed by obtaining the incident angle xcex8SP at which the intensity of the reflected light decreases.
In the conventional surface plasmon sensors as described above, the sample is analyzed based on a detected value of the total reflection break angle xcex8SP per se. Therefore, when the prism (i.e., the sample) tilts away from a predetermined position, such an inclination will produce an error in measurement of the permittivity of the sample, and this error in the permittivity will cause an error in the analysis of the sample.
In addition, a variation in a refractive index of the dielectric material or the metal film, or a deformation of the dielectric material or the metal film will also cause an error in the analysis of the sample.
An object of the present invention is to provide a surface plasmon sensor which can perform accurate analysis of a sample, even if the sample tilts, the prism or the metal film is deformed, or there is a variation in the refractive index of the prism or the metal film.
According to the first aspect of the present invention, there is provided a surface plasmon sensor which contains a dielectric material forming a prism or the like, a metal film, a light source generating a light beam, an optical system, and a light detecting unit. In the surface plasmon sensor, the light beam is led to the metal film formed on a face of the prism, and is reflected at that point. Provision is made for measuring a critical angle or a Brewster""s angle (polarizing angle), and for canceling errors in measurement of the total reflection break angle based on the critical angle or the Brewster""s angle.
To put it concretely, the surface plasmon sensor according to the present invention contains a dielectric material, a metal film formed on a face of the dielectric material and in contact with a sample, a first light source for generating a first light beam, a first optical system for letting the first light beam pass through the dielectric material and letting the first light beam be incident on a boundary surface between the dielectric material and the metal film at a first plurality of incident angles, a first light detecting unit which can detect the first light beam totally reflected by the boundary surface corresponding to the first plurality of incident angles, a second light source for generating a second light beam, a second optical system for letting the second light beam pass through the dielectric material and letting the second light beam be incident on the face of the dielectric material at a second plurality of incident angles, a second light detecting unit which can detect the second light beam regularly reflected by the face of the dielectric material corresponding to the second plurality of incident angles, a total reflection break angle obtaining unit for obtaining a total reflection break angle of the first light beam based on a result of detection by the first light detecting unit, and a critical angle obtaining unit for obtaining a critical angle of the second light beam based on a result of detection by the second light detecting unit.
Preferably, the above surface plasmon sensor may further contain a unit for obtaining a difference between the total reflection break angle and the critical angle, so that the difference can be obtained automatically and thus easily.
According to second aspect of the present invention, there is provided a surface plasmon sensor which contains a unit for obtaining the Brewster""s angle (polarizing angle) of the second light beam, instead of the critical angle obtaining unit in the first aspect of the present invention.
In the second aspect of the present invention, preferably, the surface plasmon sensor may further contain a unit for obtaining a difference between the total reflection break angle and the Brewster""s angle, so that the difference can be obtained automatically and thus easily.
In the first and second aspects of the present invention, preferably, at least light receiving portions of the first and second light detecting units may be realized by a common unit.
In the first and second aspects of the present invention, preferably, the first and second light sources may be realized by a common light source.
In the first and second aspects of the present invention, preferably, the first and second optical systems may be realized by a common optical system.